A structural inspection is not a report on damage. It is the step that establishes how much damage is actually there — and without it, every decision that follows is a guess with a signature on it. Aircraft structural inspections exist to convert a visible finding into a measured boundary, so that the condition can be compared against a published limit and a repair route can be chosen. The inspection is not a service that runs alongside the repair. It is what makes the repair defensible.
That framing matters because the visible mark is almost never the edge of the damage, and it holds for every form of aircraft structural damage a shop sees. An impact leaves a dent on the surface and cracking underneath. A composite panel absorbs a strike and shows a scuff over internal delamination. Corrosion measured at the paint line says nothing about how far it has traveled under the coating. What follows is how visual inspection, nondestructive testing and damage mapping work together to find the real boundary, and what each one can and cannot tell you.
Visual inspection is the first step in most structural damage assessments, and it is the method that decides whether any other method is needed. It identifies conditions that are apparent on the surface: dents, cracks, scratches, corrosion, impact marks, paint damage, deformation, missing fasteners, loose or damaged panels, and signs of distress around previous repairs.
It ranges from a general visual assessment to a detailed inspection using additional lighting, mirrors, access equipment, magnification or surface cleaning. The level applied depends on the structure, the damage location, the applicable maintenance instructions, the inspection task, and the approved data for that aircraft or structural component.
Visual inspection may be sufficient on its own when the applicable instructions, structural repair manual, task card or approved data confirm that the condition sits within allowable limits and requires no further inspection. Aviation authority guidance recognizes it as the predominant damage detection method used in the field, performed under prescribed lighting conditions.
Its limits are equally well established. Visual inspection may not reveal hidden cracks, internal corrosion, composite delamination, disbonding, subsurface impact damage, or damage concealed beneath coatings, fasteners or layered structures. When the damage mode cannot be characterized by looking at it, escalation is required — and that is a technical determination, not a matter of caution.
Not every visible defect requires nondestructive testing. NDT is applied when approved maintenance data, task cards, engineering instructions, structural repair manuals or operator procedures call for additional inspection, or when the suspected damage mode cannot be characterized any other way.
Escalation is typically needed when:
· hidden damage is suspected
· damage is located on or near primary structure
· cracking cannot be ruled out visually
· corrosion depth or spread cannot be confirmed visually
· impact damage may have affected composite layers
· damage sits near fastener rows or load paths
· previous repair areas are affected
· damage exceeds visual-only assessment limits
· the applicable manual requires a specific NDT method
· engineering review requires more precise characterization
A small visible mark on a composite structure may require further inspection if impact damage or internal delamination is possible. A dentin a metallic structure near fasteners or in a high-load area may require additional inspection if cracking or deformation needs to be ruled out.
The decision rule is worth stating plainly: NDT is not selected because a finding looks serious. It is selected because the suspected damage mode, the material, the structure and the applicable instructions require a method capable of characterizing that condition properly.
Nondestructive testing covers several inspection methods. The correct one depends on the material, the damage type, the geometry, the access available, the approved procedure and the sensitivity required.
Ultrasonic testing detects subsurface flaws in both metallic and composite structures. It is used to evaluate delamination, disbonding, voids, internal indications, thickness loss, corrosion-related material loss and hidden impact damage.
In composite structures it is the method of record when internal damage cannot be assessed visually. Depending on the procedure, the access and the equipment, ultrasonic inspection generally provides more detailed information than tap testing or visual inspection alone — which is why it is usually what follows a tap-test indication rather than what replaces it.
Eddy current testing is used for surface and near-surface crack detection in conductive materials such as aluminum and titanium. It is applied at fastener holes, on wing skins, at lap joints, across structural panels, in areas exposed to fatigue, and wherever crack initiation is a concern.
It is the right choice when the suspected damage mode involves surface or near-surface discontinuities in a conductive material. Its applicability depends on the material, the access, the surface condition and the approved inspection technique.
Magnetic particle inspection applies to ferromagnetic parts.It identifies surface and near-surface discontinuities in parts such as steelfittings, brackets, mounts and other ferromagnetic structural elements.
The method requires appropriate magnetization, particle application, inspection conditions and post-inspection controls in accordance with the approved procedure. Skipping any of those steps does not make the inspection faster; it makes the result unusable.
Radiographic testing detects internal conditions in specific metallic assemblies or structural components, where approved procedures, safety controls, access and equipment support its use.
It is less common in routine line maintenance because of safety, access and operational constraints. In controlled maintenance environments it remains appropriate for certain structures, castings and assemblies.
Tap testing serves as a screening method for certain composite structures. A change in sound can suggest a possible disbond or delamination.
It is not, on its own, sufficient for a return-to-service decision. Approved maintenance data, acceptance criteria, damage location and engineering requirements determine whether additional inspection — typically ultrasonic — is needed to establish what the change in sound actually represents.
Hidden damage is the reason structural inspection is a discipline rather than a checklist, and composite structure is where the gap between what is visible and what is real opens widest.
Authority guidance on composite structure states the problem directly: visual indications of damage provide limited details on the hidden parts of the damage and require further investigation. It goes further on what the follow-up has to achieve — nondestructive inspection performed before repair must be shown to locate and determine the full extent of the damage. Not to confirm that damage exists. To find where it ends.
The same guidance carries a warning that shapes how bonded repairs are controlled: certain processing defects cannot be reliably detected once the repair is complete, weak bonds among them. Where a defect cannot be found afterward, it has to be prevented beforehand — which is why in-process control during a repair is part of the inspection story, not a separate quality topic.
For metallic structure the mechanism differs and the principle holds. A smooth dent can conceal cracking initiating at a fastener hole. Corrosion visible as a paint blister can extend well beyond the blister. The inspection that matters is the one that establishes the edge.
Only once that edge is known does the next step become possible: the damage gets measured against published limits,and the route follows from which side of the limit it lands on. Every one of those comparisons inherits the accuracy of the boundary found here.
Composite and metallic structures require different inspection logic, because they fail differently.
Composite materials can hide internal damage even when the surface mark appears minor. Impact may create delamination, crushed core, disbonding or internal defects that are not visible from outside. Ultrasonic inspection or another approved method may therefore be required whenever composite impact damage, disbonding or internal distress is suspected.
Metallic structures more often require methods that detect cracks, corrosion, deformation or material loss. Eddy current testing suits conductive materials where surface or near-surface cracking is suspected. Ultrasonic testing suits thickness measurement and internal indications. Magnetic particle inspection suits ferromagnetic parts.
Method selection should always align with:
· aircraft structure
· material type
· damage mode
· accessibility
· approved maintenance data
· technician qualification
· equipment capability
· acceptance criteria
· documentation requirements
The method chosen has to be capable of characterizing the suspected damage mode under the applicable procedure. A method that cannot resolve the condition you are looking for produces a clean result and no information.
Damage mapping is the process of documenting the location, dimensions, orientation and characteristics of structural damage in a form that supports engineering and repair decisions. It is the step that turns inspection output into engineering input.
A useful damage map includes:
· photographs with a scale reference
· damage length, width and depth
· the dent or deformation profile
· proximity to fasteners
· proximity to structural joints or load paths
· the affected material or structure
· previous repairs nearby
· corrosion area and spread
· NDT results
· technician notes
· references to the applicable data
Without accurate mapping it is difficult to determine whether damage sits within allowable limits, whether a standard repair applies, whether additional inspection is required, or whether engineering review is needed. For structural repair planning, mapping is the bridge between inspection and action — and it is the part most often done thinly.
Engineering decisions depend on clear, traceable data. When damage sits close to fasteners, load paths, previous repairs, control surfaces, pressure boundaries, composite structure or primary structure, the engineering team needs precise measurements and inspection results before determining the next step.
Damage mapping supports decisions such as:
· accept as found, where approved data allows
· perform additional inspection
· repair using standard repair data
· request an engineering disposition
· seek manufacturer input
· evaluate an engineered repair approved by delegated engineering, where applicable
· remove or replace the affected structural component
· restrict operation until corrective action iscomplete
It also improves communication between inspection teams, engineering, maintenance control, repair planners and the operator. A well-documented finding reduces ambiguity. A poorly documented finding delays the decision — and the delay is usually charged to the shop, not to the report.
Once damage has been inspected and mapped, what follows depends on approved limits, available repair data, operational impact and technical feasibility.
Damage within allowable limits is handled according to the applicable structural repair manual,maintenance instruction, task card or approved procedure. Damage beyond published limits may require additional inspection, engineering review, manufacturer input, engineering data approved through delegation, repair development, replacement of the structural component, further teardown or access, operational restriction, or deferral where approved procedures allow it.
The repair-or-replace decision is not a cost question with a technical wrapper. It turns on airworthiness requirements, approved data, the capability of the organization performing the work, inspection evidence, documentation and the operator's procedures. Repair is appropriate when data, tooling, qualified personnel and inspection evidence support it. Replacement is required when damage exceeds what any available repair scheme can restore, or when the repair path cannot support the operational timeline.
What the inspection contributes to that decision is the only thing it can contribute: the true extent of the damage. Everything down stream inherits its accuracy.
Return to service depends on more than completing an inspection or a repair. The documentation package has to support both the maintenance action performed and the decision that led to it.
Depending on the task, that package may include inspection records, photographs with measurement references, NDT reports, technician qualification records, equipment calibration references, applicable task cards, structural repair manual or approved data references, the engineering disposition, repair records, material traceability, final inspection records and the release documentation.
What is required in a given case depends on the aircraft, the operator's procedures, the regulatory framework, the work scope and the approved maintenance data. For structural repairs, traceability matters because future inspections, audits, lease transitions and repeat findings all depend on understanding what was found, what was done, and what data supported the decision. Our article on structural repairtraceability(https://www.diverseaero.com/post/structural-repair-traceability) covers what a complete file looks like.
Structural inspection is not a stand-alone event. It sits inside a workflow that runs from the first finding through documentation, engineering evaluation, repair planning and return to service — and the value of doing it well shows up at the end of that chain, not at the start.
When a finding is properly characterized and properly documented, you get a decision that holds: faster to reach, easier to justify to your authority, and defensible years later when someone reviews the file during a lease transition. When it is not, the same finding generates questions that consume more time than the inspection would have.
That is the whole argument for treating inspection as the first engineering step rather than as a service line item. It is also why the inspection, the mapping and the repair route are best kept in one conversation instead of three.
Have a finding you need characterized? Send us the damage report and we will tell you what the inspection path looks like.
Visual inspection may be sufficient when the applicable maintenance instructions, structural repair manual, task card or approved data define it as acceptable and the damage sits within allowable limits.
If hidden damage, primary structure involvement, composite delamination, cracking or corrosion depth is suspected, additional inspection such as NDT is generally required.
NDT technician qualification depends on the applicable regulatory framework, employer procedures, inspection method and the industry standards used by the organization. In aviation, NDT personnel are commonly qualified under recognized standards such as NAS 410 or EN 4179, or under an employer-based written practice, depending on the program and the authority's requirements.
Operators and repair stations should confirm that technician qualification, method authorization, training records, vision requirements and equipment calibration are current and documented.
Inspection time varies with access, damage size, structural complexity, method, documentation requirements, and whether the findings trigger engineering review or further inspection. The larger variable is usually not the inspection itself but what the inspection uncovers — which is an argument for characterizing the damage early rather than in stages.
